Such an apparatus is known, inter alia as a video-disc player and as a compact-disc digital audio player, the latter being described comprehensively in "Phillips Technical Review", Vol. 40, 1982, No. 6, the entire issue. An example of, in particular the control-signal generator, is described in the previous Patent Application PHN No. 10.804 (NL No. 8303564).
In such apparatus both the information signal and the low-frequency signal for the various serve controls is derived from the signals from the photodetectors (four in said example) by amplifying the photo-detection signals through current-voltage conversion. Since the photo-currents are often very small, the amplifiers should have very low offset voltages in order to guarantee satisfactory and reliable control-signal generation, which demands either the use of low-offset amplifiers or an adjustment.